Urban and industrial communities face the serious problem of how to safely dispose pollutants. The pollutants are often deposited in wastewater, which is of several forms. It may be septage, or sewage.
Septage consists of the contents of residential and industrial septic treatment tanks. The major constituent of residential septage is solid human waste. Human waste often carries with it human pathogens, which are microorganisms that cause illness or otherwise stress humans. Human waste includes biodegradable organic matter either dissolved or suspended, which are quantified by a factor known as the biochemical oxygen demand ("BOD"). BOD is a measure of the degree to which constituents in wastewater will take up free oxygen (O.sub.2). The oxygen absorbing constituents are largely decomposing organic matter in a decomposing state. Biological methods of wastewater treatment (discussed below) depend on the presence of sufficient quantities of free oxygen in the water. Typically, biological waste treatment is conducted in part by oxygen breathing bacteria. Thus, a high BOD indicates that the waste in the water is monopolizing all of the available oxygen and "suffocating" any oxygen breathing bacteria that may be present. Septage may also include carbonaceous organic compounds (dissolved and suspended) and nitrogen, phosphorus and potassium, referred to as "nutrients" because they are necessary for the metabolism of organisms, including both microscopic and macroscopic, of the 5 living kingdoms of (plants, animals, fungi, protoctists and monera (bacteria)). Septage also includes paints, oils, cleaning fluids, heavy metals, and other toxins such as "PCBs" (polychlorinated biphenols), "teflon" (polytetrafluoroethylene) etc. A toxin is generally defined as a poison.
The wastewater may also be sewage, which has a much higher liquid content than septage, but which may contain the same types of pollutants mentioned above. Typically, septage is 30-100 times more concentrated than sewage. Typically, sewage has a BOD less than 700 and septage has a BOD of greater than 800.
Typically, sewage enters the municipal sewage system from the user's facility hookup, or through storm drains, and then proceeds to a sewage treatment center, where various treatment methods are applied. Septage, conversely, is collected by tanker trucks as part of regular maintenance when an individual septage tank needs servicing, for example, the septic system becomes clogged or otherwise unusable so that pumping is required. The septage trucks transport the septage to a waste facility and deposit the septage there for treatment.
The principal mode of conventional treatment involves separating out harmful substances from the water in various stages. Particulates are separated using mechanical processes, including filtration, centrifuging and settling. The concentrated particulates are then disposed of in a solid waste landfill. However, these particulates still include the harmful pollutants and landfill must be considered to be and treated as a hazardous waste site. Additional pollutants dissolved in the water are maintained in coarse suspension or are precipitated from the water by combination with chemicals that reduce their solubility. These additional particulates are then removed as were those in the earlier stages. Finally, pathogens, harmful organisms in the water, are killed by chlorine or other chemicals and also by exposure to ultraviolet light.
The foregoing conventional processes have many drawbacks. The mechanical processes require machinery to move large quantities of water against a pressure gradient and are thus capital intensive. Further, construction and operational costs are extremely high. Large amounts of energy must be applied to the system to effect the filtration and the segregation of the solid particulates from the water. Finally, and most distressingly, in most cases the pollutants are not changed in their basic form, and remain harmful substances. Although they are buried in landfills, they may eventually harm the environment, with the washing of rains and passage of ground water, they return to the hydrological system.
Known systems attempt to minimize the use of chemicals and emphasize the use of biological systems to remove pollutants from sewage. To date, no biological system successfully treats septage. One system passes polluted sewage water through beds of certain living plants to remove certain pollutants. Prior to treatment with the plants, the water is separated from emulsible components, such as oil or tar, which components float in a layer above the water. Coarse suspended matter settles to a sludge layer at the bottom of a settling treatment tank. A two stage living plant filtration system is typical. In the first stage, a plant such as Phragmites communis, which has roots extending from nodes, is rooted in a two layer sand bed. The root structure maintains open passageways through the sand so that the water can flow through freely. The upper layer is composed of fine sand, as compared to the lower layer. Slime and other suspended matters which are too coarse to pass through the fine sand filtration bed collect on top. After a suitable amount of slime has collected, the treatment tank is drained and dried out. The slime concentrates into a thin layer which cracks and curls up and is physically removed.
Water leaving the filtration bed contains dissolved material and pathogenic organisms. In the second stage, the water passes through a second bed of sand, in which is rooted a plant such as Scirpus lacustris. This plant removes organic compounds and inorganic ions and bacteria from the water. If necessary, additional stages using other plants that remove organic compounds, ions and bacteria not removed by Scirpus lacustris may be applied.
It is known that certain plants effectively remove particular dissolved pollutants from sewage. Scirpus lacustris (mentioned above) and Typha angustifolia remove organic aromatic compounds and pathogenic organisms, including E. coli, Salmonella, acid-fast bacteria, Ascarides and Oxyuris. They also effectively remove chemical anions, phosphates, nitrates, sulfates and chlorides.
It is also known that the roots of several species of tall growing bulrushes are effective for removing halogenated phenols such as pentachlorphenol from sewage. In harsh environments, plants must be protected by using a hot house type system. Water laden with pathogens can be purified with vegetation having certain bacterial root nodules. Bacteria live in the nodule. Many types of bacteria produce an antibiotic, which in nature protects those bacteria from other strains of bacteria by killing them. These bacteria can be used in a wastewater treatment system to kill bacteria that are harmful to humans and animals. Known systems cover the surface of the containment vessel with opaque material to prevent access of light to the water, which light makes possible the growth of algae.
This known method has many drawbacks. Principally, it produces large quantities of sludge which must be disposed of. Additionally, the slime can only be removed if the filtration bed is allowed to dry out. Thus, redundant equipment must be available to process the water while the bed is drying out. Further, removing the dried slime is a tedious and labor intensive process and also results in slime, which must be disposed.
Additional known systems for treating sewage (but not septage) use biological methods of wastewater treatment including wetland systems; aquatic plant processes; and combined aqua culture systems.
Wetland methods for sewage treatment use marshes, either natural marshes or man-made marshes. The use of natural marshes must be monitored very carefully so as not to pollute the natural environment. Wetland systems have been used successfully in pilot operations to reduce BOD, suspended solids ("SS"), trace organic compounds and trace heavy metals. However, problems with insects, such as mosquitoes, are prevalent. Further, wetland systems take up huge amounts of surface area. Further, as they mature, they become less effective. Eventually, they become so inefficient that they cannot be used.
Also known for treating sewage are aquatic plant systems where free floating aquatic plants (known as "macrophytes") are used for the treatment or refinement (sometimes referred to as "polishing") of wastewater. Water hyacinth systems may reduce BOD, SS, metals, nitrogen and refractory trace organics. Water hyacinths, however, cannot remove phosphorous in high degrees. An active mass of organisms lives in the root system of the water hyacinths. These organisms play the major role in the chemical breakup of the pollutants. In order to maintain the system, the water hyacinths must be harvested and removed. The amount of plant biomass produced in a water hyacinth pond system is about four times the quantity of waste sludge produced in conventional activated sludge secondary wastewater treatment. This plant mass must be disposed of. Disposal of the plant mass is a problem, as the plant mass will probably have incorporated the pollutants within its structure and may constitute a hazardous waste. Further, the mosquito control is very difficult with water hyacinth systems. Mosquito eating fish must be used.
The use of duckweed rather than or in addition to water hyacinth has been suggested for treating sewage, however, very little data is presented.
Combined aqua culture systems have also been proposed for treating sewage. An aqua-culture system is defined as one that produces a useful biomass from a controlled aquatic media. Examples of a useful biomass are plants that are consumable by either humans or animals. These systems, however, are unacceptably labor intensive.
A method of treating wastewater from citrus processing factories has been proposed which includes introducing the wastewater into a pool containing an absorbent material such as peat moss or shredded paper and a large quantity of earth worms. The peat moss or shredded paper absorbs the pollutants from the liquid and the worms consume the paper or peat moss. Additionally, downstream, fish that feed on the lower end of the food chain (i.e., smaller animals such as plankton), such as Tilapia, are introduced. This method may not be used to treat wastewater containing petroleum oils and/or industrial chemicals, which would destroy the worms.
An aquatic pond stocked with a large number of organisms, such as fish, snails, worms, turtles, pollywogs, bacteria, microorganisms, algae, water lilies and other vegetation has been proposed to further reduce by 80-95% the BOD already reduced by other processes. A method of protecting the aquatic pond against ruin caused by a waste overload from the sewage plant has been proposed. It provides a haven where a seed colony of the aquatic life necessary to re-populate the pond will also (and always) be present due to the constant addition of oxygen saturated water.
Another sewage treatment system has been proposed that includes a stage where solids are decomposed by the action of unspecified anaerobic bacteria, which hydrolyze and ferment complex organic compounds to simple organic acids. An additional stage is provided where the wastewater is treated with activated carbon and a mixed microbial population, which removes organic matter, organic nitrogen, ammonia (NH.sub.3) and nitrogen in the forms of nitrate (NO.sub.3) and nitrite (NO.sub.2) from the processed wastewater. However, chemicals must be used to remove phosphates and a chemical coagulant and additional chemicals such as hypochlorite are added. Ozone (O.sub.3) is also added to the wastewater, which may result in chlorinated compounds, a potential health hazard.
Another sewage treatment system has been proposed using a combination of natural ecological processes, including: an aerated lagoon; submerged, high surface area, activated bio-web substrates providing a fixed bacterial film; floating aquatic plants for nutrient (pollutant) removal; a polyculture of micro-invertebrates, fish, and shell fish in a balanced food chain for removal of nutrients and organics from the wastewater and concentration into a biomass. A solar heated greenhouse-type cover is disclosed to prevent the system from damage due to cold. Drawbacks of this system are that management is time consuming; it is difficult to maintain physical segregation between oxygenated and unoxygenated zones.
It has been disclosed that certain bacteria plasmids (small molecules of DNA) enable bacteria to degrade obnoxious halogenated organic wastes, such as chlorinated aromatic compounds.
A process for the purification of polluted water has been proposed including applying aquatic plants to the water. The method is particularly directed towards non-punctiform pollutants, such as fertilizers. The system has four different types of plant zones: a swamp zone, a marsh zone, a reed zone, and a quaking bog. This system suffers from seasonal fluctuations in functionality; senescence; excessive cleaning requirements and bad effects on ground water.
It has also been proposed to purify sewage using an expanded bed reactor containing film upon which grow methane producing anaerobic bacteria. This system must be shielded from light, because the bacteria cannot tolerate light.
All of the foregoing methods suffer from certain drawbacks, as have been discussed.